WoWDB

For the tenth anniversary celebration of World of Warcraft's launch, Molten Core will be available as a 40-man Raid Finder instance for max level characters.  Players will earn an achievement and  Core Hound Chain (mailed to your inbox) for killing the Firelord. (Source)

Players are also guaranteed an item level 640 epic helm from Ragnaros.  An important note for collectors, role players, or transmogrification enthusiasts:  The model used by the helm is normally restricted to cloth and mail armor classes.  This is your only opportunity (for now, anyway) to acquire the beautiful model for your plate or leather armor classes.

There is also a chance for players to loot a battle pet and an enchantment illusion.